Which of the following is used against external protozoan infection?

Correct option is C

Correct Answer: (c) Common salt
Explanation:
→ Common salt (NaCl) is effective against many external protozoan parasites such as Ichthyophthirius, Trichodina, and Chilodonella.
→ It dehydrates protozoa and disrupts osmotic balance.
Information Booster:
→ Widely used because it is cheap, safe, and non-toxic at proper dosage.
Additional Information:
(a) Methylene blue: Effective but mainly for fungal infections.
(b) Oxytetracycline: Antibiotic, not anti-protozoal.
(d) Paracetamol: Not used in aquaculture.